1. Field
Example embodiments relate to a semiconductor memory device, and for example, to an over driver of a semiconductor memory device and/or an over driving method thereof.
2. Description of Related Art
As semiconductor memory devices become more highly integrated and have lower-power consumption, a lower driving voltage and a higher-speed operation are required. As drive voltages of the semiconductor memory devices have been gradually lowered, several technical solutions for assisting an operation of a sense amplifier in the semiconductor memory devices have been proposed. One of the proposed solutions is a method of over-driving the sense amplifier.
The sense amplifier amplifies a voltage difference between a pair of bit lines. Therefore, a data read/write operation and a data refresh operation of a memory cell may be performed. The operations of the sense amplifier are controlled by a sense amplifier controller.
In a conventional sense amplifier, a P-channel metal oxide silicon (PMOS) transistor has a voltage-driving capability that is relatively lower than that of an N-channel metal oxide silicon (NMOS) transistor. That is, the voltage driving capability of the PMOS transistor is about twice as high as that of the NMOS transistor. However, with an increase in a memory capacity of a DRAM, a chip size of the DRAM becomes smaller. Accordingly, the size of the PMOS transistor in the sense amplifier is substantially equal to that of the NMOS transistor. As a result, the data-driving capability of the PMOS transistor is reduced. In particular, a limitation occurs if bit line data is amplified to a higher voltage in the sense amplifier. In order to resolve this limitation, a conventional method over-drives a sense amplifier to over-drive sensing data if the sense amplifier is enabled.
If an over-driven sense amplifier is activated, an external voltage VEXT, e.g., about 3.3V, is applied to enhance a data sensing speed. Thereafter, an array internal voltage VINTA, e.g., about 2.2V, which is adjusted lower for maintaining an accumulated voltage, is applied to the sense amplifier. A target voltage (i.e., the internal voltage VINTA) takes a relatively longer time to reach while a precharge voltage VBL is pulled up to the internal voltage VINTA because a difference voltage between the internal voltage VINTA, e.g., about 2.2V, and the precharge voltage VBL is relatively not that great, i.e, relatively smaller. In order to overcome this limitation, the external voltage VEXT, e.g., about 3.3V, is applied during a desired, or alternatively, a predetermined period. A method of applying the external voltage VEXT during the desired, or alternatively, the predetermined period is called an “over driving method.”
A conventional over driving method provides the external voltage VEXT through a power line of a mesh structure by each bank unit. However, the power line of the mesh structure causes vulnerable nodes in respective regions if considering resistance capacitance (RC) delay. As a result, an over driving effect of the sense amplifier is remarkably reduced in the vulnerable nodes.